Analysis of Parallel Operation of Uninterruptible Power Supplies Loaded through Long Wiring Cables

This paper discusses and analyses the parallel operation of uninterruptible power supplies (UPS) units when loaded through long wiring cables having non-negligible inductance and resistance. This scenario is often encountered in practice, as the UPS power supplies are often located far from the load they are actually powering. The analysis demonstrates how long cable connections between the power supply system and the load deeply modify the behavior of the single UPS unit. More precisely, it is shown how the cable inductance interacts with the UPS closed-loop output impedance modifying the control loop gain, ultimately generating resonance phenomena which deteriorate or even compromise system stability and performances if suitable provisions are not taken at the design stage. Simulation and experimental results carried out on two paralleled three-phase, 40 kVA UPS units based on voltage-source inverters topologies confirm the validity of the theoretical discussion and the practical relevance of the phenomenon.